Strong coffee beans, and which kind of strong you are buying

The strongest coffee beans by caffeine are the ones made mostly or entirely of robusta, which carries far more caffeine than arabica: Black Donkey Devil's Roast is a 100% robusta kilo at around 32p a cup, and Nam Song Dak Lak Robusta is a single-origin robusta from Vietnam. If strong means a dark, smoky flavour with some extra caffeine, Taylors of Harrogate Hot Lava Java is the darkest roast Taylors makes, with robusta added for the caffeine.

Which coffee beans are the strongest?

If you mean caffeine, the ones with the most robusta in them. Robusta beans contain 2.7% caffeine against arabica's 1.5%, on the figures in Wikipedia's entry for the species, and Taylors of Harrogate's coffee team puts it the same way: arabica is around 1.5% caffeine by weight and robusta has 'almost twice as much'. A 2014 study in the journal Planta measured a smaller gap in mature beans, 50% more caffeine in robusta. Either way, an 18g dose of all-robusta beans carries between half as much again and nearly twice the caffeine of an all-arabica dose.

If you mean flavour, the darkest roasts: smoke, bitterness and body. Those are different things, and the three picks below cover both.

Black Donkey Devil's Roast: most caffeine for the money

Black Donkey describes Devil's Roast as a '100% premium robusta blend', dark roasted in Italy, and warns that it is 'a highly caffeinated brew, do not drink when you need to sleep!' On Black Donkey's own roast strength chart it scores 11, the top of the range: Intenso, which is 80% robusta, scores 8, and the Colombian arabica Delicato 7. Black Donkey's tasting note is short: low acidity, with nutty chocolate.

It cost around 32p a cup when we last checked, about half the 63p median for whole bean coffee. The listing is vague about where the beans grow. It names Colombia, Brazil, Kenya, Ethiopia and India for Black Donkey's beans in general, and the comparison chart calls Devil's Roast a natural robusta blend. Nobody independent appears to have reviewed it.

Skip it if you drink coffee black and want it sweet, or if you are sensitive to caffeine in the afternoon. Robusta is usually more bitter than arabica, and this is all robusta.

Black Donkey Intenso, the 80% robusta kilo a penny a cup cheaper, is reviewed in full, including what the ratio does in milk and in a bean-to-cup machine. Black Donkey Intenso review

Taylors of Harrogate Hot Lava Java: strongest flavour

Taylors calls Hot Lava Java an extra-dark roast with 'hints of smoke and black pepper' and rates it 6 for roast, which its Amazon listing describes as Taylors' darkest roast. The blend is led by Indonesian beans, with India and Brazil in support, and Taylors' own page explains the rest: 'Unusually for Taylors, this doesn't consist entirely of arabica coffee. To give that extra wallop of caffeine, we've included some high-quality robusta, too.'

The Amazon listing for the beans contradicts that, describing a '100% ground arabica' blend on a page for whole beans. Taylors' own site is closer to the coffee, so this page goes with robusta. Taylors does not publish how much robusta is in it, so its caffeine sits somewhere between an all-arabica bag and Devil's Roast.

It comes as six 200g bags, which suits a strong coffee you may not drink every day, since each bag stays sealed until you need it. It cost around 40p a cup when we last checked. Skip it if smoke and bitterness are what you dislike about dark roasts.

Union Revelation is a dark espresso blend that is all arabica, if you want the depth without the robusta. Union Revelation review

Nam Song Dak Lak Robusta: best single origin

Nam Song's beans are robusta grown in Dak Lak, in Vietnam's Central Highlands, at 1,000 to 1,600 metres, and roasted dark in Sheffield. Nam Song describes molasses and toffee, and says robusta 'offers a stronger taste and higher caffeine content than standard blends'. The coffee is designed for the Vietnamese phin filter, a slow-drip brewer; as whole beans, you can grind them for a moka pot or espresso too.

Vietnam is where robusta comes from in volume: Taylors' coffee team notes that about 40% of the world's coffee is robusta and that most of it is grown in Vietnam and Indonesia. The 500g bag cost around 65p a cup when we last checked, 2p over the median, which is the price of a single origin and a smaller bag. There is no independent review of it yet.

Skip it if you want the most caffeine for the least money, which is Devil's Roast at half the price per cup.

How do the three compare?

Two are all robusta, so they carry the most caffeine; Hot Lava Java has the darkest flavour and an unstated share of robusta. Devil's Roast is the cheapest by 8p a cup.

At two cups a day, 730 doses, a year costs about £234 for Devil's Roast, £292 for Hot Lava Java and £475 for the Nam Song, against £460 at the median. The cards at the top carry today's prices.

Does dark roast coffee have more caffeine?

No. Caffeine levels are not significantly affected by the roast, according to Wikipedia's article on roasting, which also explains the myth's other half. Beans lose 15 to 18% of their mass in the roaster, mostly water, while their volume roughly doubles. So measured by the scoop, a light roast holds slightly more caffeine than a dark one, because the dark beans are less dense. Our per-cup prices use an 18g dose by weight, where the difference is small.

What a dark roast does give you is the taste people call strong. Union Hand-Roasted describes darker roasts as 'fuller bodied, with bold, rich chocolatey notes', and Taylors sells Hot Lava Java on smoke and black pepper. That is the kind of strong Hot Lava Java is for.

What do intensity and strength numbers on a bag mean?

Each brand's own rating of flavour or roast, on its own scale. None of these brands says its number measures caffeine, and decaf coffees carry high numbers too.

So read the beans line, not the number. 'Robusta' or 'arabica and robusta' tells you about caffeine; the roast level tells you about flavour; the intensity score tells you how the brand ranks its own range.

Grano Milano sells its decaf as an extra strong dark roast, rated 5 out of 5 for intensity, with almost none of the caffeine. Grano Milano Decaf French Roast review

Who should skip all three?

• Anyone who wants strength without bitterness. Robusta is usually more bitter than arabica, and all three picks lean on robusta or a very dark roast; a medium-dark arabica espresso blend is the gentler route.
• Anyone pregnant or limiting caffeine. The NHS says not to have more than 200mg of caffeine a day in pregnancy, and an all-robusta coffee gets there faster than most.
• Bean-to-cup owners who want the darkest flavour. Machine makers such as De'Longhi warn against oily beans, and Hot Lava Java is an extra-dark roast, so check its surface before it goes in the hopper.

The verdict

Buy Black Donkey Devil's Roast if caffeine per penny is the point. Buy Taylors of Harrogate Hot Lava Java if you want a coffee that tastes strong, smoke and pepper included, with some robusta behind it. Buy Nam Song Dak Lak Robusta if you want a robusta that tastes of one place, or own a phin.